Reflective polarizers are commonly used to enhance the brightness of liquid crystal (LC) displays and display systems. The LC display system typically includes an LC panel, behind which is an illumination assembly or backlight positioned to provide light to the LC panel. Brightness enhancement is provided by the reflective polarizer as the result of a light recycling process: light that cannot (because of its polarization state) contribute to the display output is reflected by the reflective polarizer back into the backlight, where some of the light is re-reflected towards the reflective polarizer in a different polarization state that can contribute to the display output and that passes through the reflective polarizer toward the user or viewer.
The LC panel includes a layer of liquid crystal material disposed between glass panel plates. Furthermore, the LC panel is sandwiched between two absorbing polarizer films: a front absorbing polarizer, attached to the front glass plate of the LC panel, and a back absorbing polarizer, attached to the back glass plate. The brightness-enhancing reflective polarizer is placed somewhere behind the LC panel, and behind the back absorbing polarizer.
In practice, design details of the reflective polarizer have an impact on exactly where the reflective polarizer can be placed in the display system to provide optimal, or at least acceptable, optical performance. Some types of reflective polarizers can be laminated directly to the exposed rear surface of the back absorbing polarizer. Those of ordinary skill in the art consider it necessary for these types of reflective polarizers to have a very low perceived color for the pass state of polarization both at normal incidence (light propagating along the optical axis of the display system) and at highly oblique incidence. Since the reflective polarizer is attached to the back absorbing polarizer, and the back absorbing polarizer is in turn commonly attached to the back glass plate of the LC panel, this is referred to as an “on-glass” configuration of the reflective polarizer. One reflective polarizer currently used in the on-glass configuration is a parabolically-stretched reflective polarizer, discussed further below. Another reflective polarizer used in the on-glass configuration is a multi-packet reflective polarizer, also discussed below.
Other types of reflective polarizers, considered by those of ordinary skill in the art to have excessive perceived color for the pass state of polarization for obliquely incident light, are not laminated to the back absorbing polarizer of the display because the (undesirable) color associated with the reflective polarizer would be visible to the user through the absorbing polarizer and through the LC display. Instead, these latter types of reflective polarizers are used in the display system as a stand-alone film, separated from the back absorbing polarizer by at least one air gap, and attached to a light diffusing film or layer that is disposed between the reflective polarizer and the back absorbing polarizer. The light diffusing layer has a significant haze value so as to effectively combine light rays that pass through the reflective polarizer in different directions, to reduce or eliminate the color associated with the reflective polarizer from the standpoint of the user or viewer.